A family of Microsoft word processing software products for creating web, email, and print documents.
OK, now I see what you mean. I believe this is related to a bug reported earlier... Lines that are perfectly Vertical or perfectly Horizontal aren't selectable at all or can be very difficult to select. Note that the uppermost diagonal line in your example document is selected but the lines which are horizontal are not. Copy the link to this conversation, then while that document is displayed please use the ☺︎ at the right end of the title bar to report this & paste in the link you copied. Make sure to check the box to Include Screen Shot.
As an interim workaround: Slightly rotate any Vertical or Horizontal lines then rotate them back to their original orientation [don't use Undo].